For a 15KW microinverter grid-tie system, is battery backup even possible

[Hedges]

I think you're missing a few 000's in your kWh figures.

If the well pump doesn't exist yet, you can get a Grudfos that runs direct off PV panels, or just inverter drive for zero surge.

You can go SMA Sunny Island like mine (2s2p split-phase) or maybe 3-phase. How does 44kW or 33kW surge capability sound?

 

[opticalcarrier]

great tip on pump/inverter. Im going with the solark12k, getting AltE to get me a system together now. My builder said the roofers can do the racking and DC wiring layout for me. Ill just have to be really careful on the roof putting the panels up but I should have about 10K of power.
ill start on the battery cells in a few months after all moved in thanks everyone on the tips!

 

[Hedges]

OK, so is that Sol Ark with DC coupled PV instead of the microinverters you first considered?

Its inverter wattage is a bit less than what it can pass through to the grid. I think it supports GT inverters with frequency-watts on its output, so you can backfeed a bit more when grid tied and use that power as well while running off-grid.

 

[GXMnow]

Saw your message come in as I started typing. The Sol-Ark is getting excellent reviews. Are you just going DC coupled then, or will you stick with the Microinverters?

1800 KWH a month is about 60 KWH a day. That is only a little more than I use in the hot summer with my A/C blowing cold at 114 F outside. During the cool winter, I am only using 20 KWH a day. How often do you have a grid power failure? How long do they last? How much of your loads will need to be available during a grid failure? Do you also want to load shift when the grid is up? The typical "Off Grid" rule is to have enough battery to run your loads for 2 days. That way a day of heavy clouds can be covered. But if you are on grid, you can cut it much lower, but still plan for 2 days of the things you absolutely need to run during a grid failure. My battery bank is 17 KWH of usable power. That will easily run my essentials for 2 day, but not my air conditioning.

 

[opticalcarrier]

Im going to just forego microinverters/optimizers, Ill be segmented into 4 strings, 2 per MPPT on the solark, so thatll just have to do.
Short outages for a minute or so every month or so. But since ill be in new place with a new POCO, who knows there. Ill put in as big a battery I can until it scares me or get tired of spending$, maybe around 20KWH if I can use the new 310AH cells.. Im planning out a separate critical loads panel, prob just masterbed/bath, kitchen, and LR. I should be able to do a bit of load shifting there, not too much but enough to make a difference.

 

[Hedges]

If your building codes require "Rapid Shutdown" then there will still be a module per panel for that. ArthurEld has pictures & schematic of his system with Sol Ark and rapid shutdown.

I like to keep battery as small as possible because I consider it mostly wasted money. If you do load shifting then it may pay for itself.
For my system and the batteries I selected, PV is a fraction the cost of batteries, so oversize PV helps it work with less battery. I power A/C direct from PV --> AC while keeping batteries charged.
You might add AC coupled GT PV if you max out DC coupled. That could support loads in times of less sun while drawing less from batteries. That could be a future addition.

 

[GXMnow]

Before I found the deal on the Chevy Bolt cells I am using now, I almost bought 16 of the 280 AH metal can LFP cells. That works out to a bit over 14 KWHs. My Bolt based pack works out to 360 AH, or almost 18 KWH. You can use most of the rated capacity of Lithium cells. I am only cycling 7 to 8 KWH each day when the grid is up. That covers my whole house during the peak rate time of use. I may push it to 10 or 12 KWH if I have it help run the A/C. I want to leave at least 30% or so incase the is a late night power outage after it has been running the house during the peak rate time. Since you will be DC coupled, you have one less issue than I do. Even if you run the batteries dead, the sun will start it up the next morning. With AC coupling, I have to have the inverter running on batteries first, for over 5 minutes, before the solar inverters can start making power to help with loads or charge the battery. If my battery is dead, I am stuck. To sort of get around the problem, I do have the shut down level set up near 25% charge, so I can always log into the inverter from a laptop and set the cut off voltage lower to get it to start up and hopefully have the microinverters charge it up again.

 

[goldenfab]

Hi @opticalcarrier, I am trying to figure out the best inverter(s) for my needs and have just about the same requirements as you. Did you get your system up and running? Anything you would do different?

 

[DCPower]

Building a new home its got a massive roof over the garage, facing south. after the firefighter 3ft walkways there will be 41ft wide X 20ft high area for panels. I estimate I can max that out at about 15KW with some 440W panels. I'm planning it for a grid tie, and really want *at least some* battery backup, if only to allow me to use solar in event of outage. Please let me know how accurate the following is, regarding sizing of the battery inverter or charge controller:

I think that if I use microinverters that I'll need a separate centralized inverter for the battery, and it will need to be sized at least as much KW in solar? I think that if I use a string inverter instead of microinverters id then need to terminate the PV modules onto a battery controller, which again needs to be sized appropriately.

So, how to handle batteries on a PV system at 15KW?

and finally, why can grid-tie systems be designed to just disco off the grid when its down, and then pump the solar to the home? Seems stupid easy to design a transfer switch that cuts grid off the house when live current is off?

All of the “solutions” I’ve seen to add solar array charged battery back up capability to enphase grid tied AC module arrays have been - pardon my French - creative clusterfcks. Even enphase’s own recent attempt.

 

[GXMnow]

All of the “solutions” I’ve seen to add solar array charged battery back up capability to enphase grid tied AC module arrays have been - pardon my French - creative clusterfcks. Even enphase’s own recent attempt.

If all I wanted was battery backup, my setup with the Schneider XW-Pro does work great with the Enphase microinverters. During our last grid outage, the system performed great. A few of my super cheap LED replacement light bulbs had a slight flickering to them, but it was not bad. The better quality bulbs and all my equipment worked perfectly. I saw the battery current go from powering the house to charging as the sun was coming up. For that use case, the system is rock solid.

My only issue is the people at Schneider refuse to listen to end users about adding or fixing how some features work. With only the Enphase solar AC coupled into the system, it just refuses to start a charge cycle on it's own. All it would need is a line of code that tells it to start a bulk charge when the charge block time ends. During the last power outage, I had my charge current set low, and I saw the frequency shift kick in and 5 of my Enphase inverters did shut off, but then came back on in just 1 minute, not 5. So it held the charge current pretty close to my setting whle still running all of the loads on my backup panel. So it does work.

 

[edseld]

You can do AC coupled with backup. A 15KW Sigineer inverter will do the trick.
This would be an off grid setup. You can connect the Grid to the AC input on the inverter and if your solar and batteries go off line, it would switch over to the grid.

You can do AC coupled with backup. A 15KW Sigineer inverter will do the trick.
This would be an off grid setup. You can connect the Grid to the AC input on the inverter and if your solar and batteries go off line, it would switch over to the grid.

If you go this route you still need to find a way to disconnect the PV solar input when your battery is full. I'm thinking to go this route too, just finding a solid solution which is affordable in my pocket. I'm thinking to use relay/contactor method or simply designing my own CAN bus controller.

 

[magic8192]

If you have CA rule 21 grid tie inverters they may work off grid. I have never tried this, but CA rule 21 is to have your inverters automatically lower their output as Hz increase on the grid. https://diysolarforum.com/threads/enphase-ac-to-hybrid-system.12611/post-166434

 

[edseld]

If you have CA rule 21 grid tie inverters they may work off grid. I have never tried this, but CA rule 21 is to have your inverters automatically lower their output as Hz increase on the grid. https://diysolarforum.com/threads/enphase-ac-to-hybrid-system.12611/post-166434

yeah, maybe that feature is now available for those with micro inverters. For 15KW Sigineer, they don't have that auto setting.